Tag Archives: entanglement

Non-Spooky Action at a Distance

Albert Einstein famously called quantum entanglement “spooky action at a distance”. It refers to the notion that measuring the state of one of two entangled particles makes the state of the second particle known instantaneously, regardless of the distance  separating the two particles. Entanglement seems to link these particles and make them behave as one system. This peculiar characteristic has been a core element of the counterintuitiive world of quantum theory. Yet while experiments have verified this spookiness, other theorists maintain that both theory and experiment are flawed, and that a different interpretation is required. However, one such competing theory — the many worlds interpretation — makes equally spooky predictions.

From ars technica:

Quantum nonlocality, perhaps one of the most mysterious features of quantum mechanics, may not be a real phenomenon. Or at least that’s what a new paper in the journal PNAS asserts. Its author claims that nonlocality is nothing more than an artifact of the Copenhagen interpretation, the most widely accepted interpretation of quantum mechanics.

Nonlocality is a feature of quantum mechanics where particles are able to influence each other instantaneously regardless of the distance between them, an impossibility in classical physics. Counterintuitive as it may be, nonlocality is currently an accepted feature of the quantum world, apparently verified by many experiments. It’s achieved such wide acceptance that even if our understandings of quantum physics turn out to be completely wrong, physicists think some form of nonlocality would be a feature of whatever replaced it.

The term “nonlocality” comes from the fact that this “spooky action at a distance,” as Einstein famously called it, seems to put an end to our intuitive ideas about location. Nothing can travel faster than the speed of light, so if two quantum particles can influence each other faster than light could travel between the two, then on some level, they act as a single system—there must be no real distance between them.

The concept of location is a bit strange in quantum mechanics anyway. Each particle is described by a mathematical quantity known as the “wave function.” The wave function describes a probability distribution for the particle’s location, but not a definite location. These probable locations are not just scientists’ guesses at the particle’s whereabouts; they’re actual, physical presences. That is to say, the particles exist in a swarm of locations at the same time, with some locations more probable than others.

A measurement collapses the wave function so that the particle is no longer spread out over a variety of locations. It begins to act just like objects we’re familiar with—existing in one specific location.

The experiments that would measure nonlocality, however, usually involve two particles that are entangled, which means that both are described by a shared wave function. The wave function doesn’t just deal with the particle’s location, but with other aspects of its state as well, such as the direction of the particle’s spin. So if scientists can measure the spin of one of the two entangled particles, the shared wave function collapses and the spins of both particles become certain. This happens regardless of the distance between the particles.

The new paper calls all this into question.

The paper’s sole author, Frank Tipler, argues that the reason previous studies apparently confirmed quantum nonlocality is that they were relying on an oversimplified understanding of quantum physics in which the quantum world and the macroscopic world we’re familiar with are treated as distinct from one another. Even large structures obey the laws of quantum Physics, Tipler points out, so the scientists making the measurements must be considered part of the system being studied.

It is intuitively easy to separate the quantum world from our everyday world, as they appear to behave so differently. However, the equations of quantum mechanics can be applied to large objects like human beings, and they essentially predict that you’ll behave just as classical physics—and as observation—says you will. (Physics students who have tried calculating their own wave functions can attest to this). The laws of quantum physics do govern the entire Universe, even if distinctly quantum effects are hard to notice at a macroscopic level.

When this is taken into account, according to Tipler, the results of familiar nonlocality experiments are altered. Typically, such experiments are thought to involve only two measurements: one on each of two entangled particles. But Tipler argues that in such experiments, there’s really a third measurement taking place when the scientists compare the results of the two.

This third measurement is crucial, Tipler argues, as without it, the first two measurements are essentially meaningless. Without comparing the first two, there’s no way to know that one particle’s behavior is actually linked to the other’s. And crucially, in order for the first two measurements to be compared, information must be exchanged between the particles, via the scientists, at a speed less than that of light. In other words, when the third measurement is taken into account, the two particles are not communicating faster than light. There is no “spooky action at a distance.”

Tipler has harsh criticism for the reasoning that led to nonlocality. “The standard argument that quantum phenomena are nonlocal goes like this,” he says in the paper. “(i) Let us add an unmotivated, inconsistent, unobservable, nonlocal process (collapse) to local quantum mechanics; (ii) note that the resulting theory is nonlocal; and (iii) conclude that quantum mechanics is [nonlocal].”

He’s essentially saying that scientists are arbitrarily adding nonlocality, which they can’t observe, and then claiming they have discovered nonlocality. Quite an accusation, especially for the science world. (The “collapse” he mentions is the collapse of the particle’s wave function, which he asserts is not a real phenomenon.) Instead, he claims that the experiments thought to confirm nonlocality are in fact confirming an alternative to the Copenhagen interpretation called the many-worlds interpretation (MWI). As its name implies, the MWI predicts the existence of other universes.

The Copenhagen interpretation has been summarized as “shut up and measure.” Even though the consequences of a wave function-based world don’t make much intuitive sense, it works. The MWI tries to keep particles concrete at the cost of making our world a bit fuzzy. It posits that rather than becoming a wave function, particles remain distinct objects but enter one of a number of alternative universes, which recombine to a single one when the particle is measured.

Scientists who thought they were measuring nonlocality, Tipler claims, were in fact observing the effects of alternate universe versions of themselves, also measuring the same particles.

Part of the significance of Tipler’s claim is that he’s able to mathematically derive the same experimental results from the MWI without use of nonlocality. But this does not necessarily make for evidence that the MWI is correct; either interpretation remains consistent with the data. Until the two can be distinguished experimentally, it all comes down to whether you personally like or dislike nonlocality.

Read the entire article here.

The Arrow of Time

Arthur_Stanley_EddingtonEinstein’s “spooky action at a distance” and quantum information theory (QIT) may help explain the so-called arrow of time — specifically, why it seems to flow in only one direction. Astronomer Arthur Eddington first described this asymmetry in 1927, and it has stumped theoreticians ever since.

At a macro-level the classic and simple example is that of an egg breaking when it hits your kitchen floor: repeat this over and over, and it’s likely that the egg will always make for a scrambled mess on your clean tiles, but it will never rise up from the floor and spontaneously re-assemble in your slippery hand. Yet at the micro-level, physicists know their underlying laws apply equally in both directions. Enter two new tenets of the quantum world that may help us better understand this perplexing forward flow of time: entanglement and QIT.

From Wired:

Coffee cools, buildings crumble, eggs break and stars fizzle out in a universe that seems destined to degrade into a state of uniform drabness known as thermal equilibrium. The astronomer-philosopher Sir Arthur Eddington in 1927 cited the gradual dispersal of energy as evidence of an irreversible “arrow of time.”

But to the bafflement of generations of physicists, the arrow of time does not seem to follow from the underlying laws of physics, which work the same going forward in time as in reverse. By those laws, it seemed that if someone knew the paths of all the particles in the universe and flipped them around, energy would accumulate rather than disperse: Tepid coffee would spontaneously heat up, buildings would rise from their rubble and sunlight would slink back into the sun.

“In classical physics, we were struggling,” said Sandu Popescu, a professor of physics at the University of Bristol in the United Kingdom. “If I knew more, could I reverse the event, put together all the molecules of the egg that broke? Why am I relevant?”

Surely, he said, time’s arrow is not steered by human ignorance. And yet, since the birth of thermodynamics in the 1850s, the only known approach for calculating the spread of energy was to formulate statistical distributions of the unknown trajectories of particles, and show that, over time, the ignorance smeared things out.

Now, physicists are unmasking a more fundamental source for the arrow of time: Energy disperses and objects equilibrate, they say, because of the way elementary particles become intertwined when they interact — a strange effect called “quantum entanglement.”

“Finally, we can understand why a cup of coffee equilibrates in a room,” said Tony Short, a quantum physicist at Bristol. “Entanglement builds up between the state of the coffee cup and the state of the room.”

Popescu, Short and their colleagues Noah Linden and Andreas Winter reported the discovery in the journal Physical Review E in 2009, arguing that objects reach equilibrium, or a state of uniform energy distribution, within an infinite amount of time by becoming quantum mechanically entangled with their surroundings. Similar results by Peter Reimann of the University of Bielefeld in Germany appeared several months earlier in Physical Review Letters. Short and a collaborator strengthened the argument in 2012 by showing that entanglement causes equilibration within a finite time. And, in work that was posted on the scientific preprint site arXiv.org in February, two separate groups have taken the next step, calculating that most physical systems equilibrate rapidly, on time scales proportional to their size. “To show that it’s relevant to our actual physical world, the processes have to be happening on reasonable time scales,” Short said.

The tendency of coffee — and everything else — to reach equilibrium is “very intuitive,” said Nicolas Brunner, a quantum physicist at the University of Geneva. “But when it comes to explaining why it happens, this is the first time it has been derived on firm grounds by considering a microscopic theory.”

If the new line of research is correct, then the story of time’s arrow begins with the quantum mechanical idea that, deep down, nature is inherently uncertain. An elementary particle lacks definite physical properties and is defined only by probabilities of being in various states. For example, at a particular moment, a particle might have a 50 percent chance of spinning clockwise and a 50 percent chance of spinning counterclockwise. An experimentally tested theorem by the Northern Irish physicist John Bell says there is no “true” state of the particle; the probabilities are the only reality that can be ascribed to it.

Quantum uncertainty then gives rise to entanglement, the putative source of the arrow of time.

When two particles interact, they can no longer even be described by their own, independently evolving probabilities, called “pure states.” Instead, they become entangled components of a more complicated probability distribution that describes both particles together. It might dictate, for example, that the particles spin in opposite directions. The system as a whole is in a pure state, but the state of each individual particle is “mixed” with that of its acquaintance. The two could travel light-years apart, and the spin of each would remain correlated with that of the other, a feature Albert Einstein famously described as “spooky action at a distance.”

“Entanglement is in some sense the essence of quantum mechanics,” or the laws governing interactions on the subatomic scale, Brunner said. The phenomenon underlies quantum computing, quantum cryptography and quantum teleportation.

The idea that entanglement might explain the arrow of time first occurred to Seth Lloyd about 30 years ago, when he was a 23-year-old philosophy graduate student at Cambridge University with a Harvard physics degree. Lloyd realized that quantum uncertainty, and the way it spreads as particles become increasingly entangled, could replace human uncertainty in the old classical proofs as the true source of the arrow of time.

Using an obscure approach to quantum mechanics that treated units of information as its basic building blocks, Lloyd spent several years studying the evolution of particles in terms of shuffling 1s and 0s. He found that as the particles became increasingly entangled with one another, the information that originally described them (a “1” for clockwise spin and a “0” for counterclockwise, for example) would shift to describe the system of entangled particles as a whole. It was as though the particles gradually lost their individual autonomy and became pawns of the collective state. Eventually, the correlations contained all the information, and the individual particles contained none. At that point, Lloyd discovered, particles arrived at a state of equilibrium, and their states stopped changing, like coffee that has cooled to room temperature.

“What’s really going on is things are becoming more correlated with each other,” Lloyd recalls realizing. “The arrow of time is an arrow of increasing correlations.”

The idea, presented in his 1988 doctoral thesis, fell on deaf ears. When he submitted it to a journal, he was told that there was “no physics in this paper.” Quantum information theory “was profoundly unpopular” at the time, Lloyd said, and questions about time’s arrow “were for crackpots and Nobel laureates who have gone soft in the head.” he remembers one physicist telling him.

“I was darn close to driving a taxicab,” Lloyd said.

Advances in quantum computing have since turned quantum information theory into one of the most active branches of physics. Lloyd is now a professor at the Massachusetts Institute of Technology, recognized as one of the founders of the discipline, and his overlooked idea has resurfaced in a stronger form in the hands of the Bristol physicists. The newer proofs are more general, researchers say, and hold for virtually any quantum system.

“When Lloyd proposed the idea in his thesis, the world was not ready,” said Renato Renner, head of the Institute for Theoretical Physics at ETH Zurich. “No one understood it. Sometimes you have to have the idea at the right time.”

Read the entire article here.

Image: English astrophysicist Sir Arthur Stanley Eddington (1882–1944). Courtesy: George Grantham Bain Collection (Library of Congress).

Spooky Action at a Distance Explained

[div class=attrib]From Scientific American:[end-div]

Quantum entanglement is such a mainstay of modern physics that it is worth reflecting on how long it took to emerge. What began as a perceptive but vague insight by Albert Einstein languished for decades before becoming a branch of experimental physics and, increasingly, modern technology.

Einstein’s two most memorable phrases perfectly capture the weirdness of quantum mechanics. “I cannot believe that God plays dice with the universe” expressed his disbelief that randomness in quantum physics was genuine and impervious to any causal explanation. “Spooky action at a distance” referred to the fact that quantum physics seems to allow influences to travel faster than the speed of light. This was, of course, disturbing to Einstein, whose theory of relativity prohibited any such superluminal propagation.

These arguments were qualitative. They were targeted at the worldview offered by quantum theory rather than its predictive power. Niels Bohr is commonly seen as the patron saint of quantum physics, defending it against Einstein’s repeated onslaughts. He is usually said to be the ultimate winner in this battle of wits. However, Bohr’s writing was terribly obscure. He was known for saying “never express yourself more clearly than you are able to think,” a motto which he adhered to very closely. His arguments, like Einstein’s, were qualitative, verging on highly philosophical. The Einstein-Bohr dispute, although historically important, could not be settled experimentally—and the experiment is the ultimate judge of validity of any theoretical ideas in physics. For decades, the phenomenon was all but ignored.

All that changed with John Bell. In 1964 he understood how to convert the complaints about “dice-playing” and “spooky action at a distance” into a simple inequality involving measurements on two particles. The inequality is satisfied in a world where God does not play dice and there is no spooky action. The inequality is violated if the fates of the two particles are intertwined, so that if we measure a property of one of them, we immediately know the same property of the other one—no matter how far apart the particles are from each other. This state where particles behave like twin brothers is said to be entangled, a term introduced by Erwin Schrödinger.

[div class=attrib]Read the whole article here.[end-div]

Quantum Trickery: Testing Einstein’s Strangest Theory

[div class=attrib]From the New York Times:[end-div]

Einstein said there would be days like this.

This fall scientists announced that they had put a half dozen beryllium atoms into a “cat state.”

No, they were not sprawled along a sunny windowsill. To a physicist, a “cat state” is the condition of being two diametrically opposed conditions at once, like black and white, up and down, or dead and alive.

These atoms were each spinning clockwise and counterclockwise at the same time. Moreover, like miniature Rockettes they were all doing whatever it was they were doing together, in perfect synchrony. Should one of them realize, like the cartoon character who runs off a cliff and doesn’t fall until he looks down, that it is in a metaphysically untenable situation and decide to spin only one way, the rest would instantly fall in line, whether they were across a test tube or across the galaxy.

The idea that measuring the properties of one particle could instantaneously change the properties of another one (or a whole bunch) far away is strange to say the least – almost as strange as the notion of particles spinning in two directions at once. The team that pulled off the beryllium feat, led by Dietrich Leibfried at the National Institute of Standards and Technology, in Boulder, Colo., hailed it as another step toward computers that would use quantum magic to perform calculations.

But it also served as another demonstration of how weird the world really is according to the rules, known as quantum mechanics.

The joke is on Albert Einstein, who, back in 1935, dreamed up this trick of synchronized atoms – “spooky action at a distance,” as he called it – as an example of the absurdity of quantum mechanics.

“No reasonable definition of reality could be expected to permit this,” he, Boris Podolsky and Nathan Rosen wrote in a paper in 1935.

Today that paper, written when Einstein was a relatively ancient 56 years old, is the most cited of Einstein’s papers. But far from demolishing quantum theory, that paper wound up as the cornerstone for the new field of quantum information.

Nary a week goes by that does not bring news of another feat of quantum trickery once only dreamed of in thought experiments: particles (or at least all their properties) being teleported across the room in a microscopic version of Star Trek beaming; electrical “cat” currents that circle a loop in opposite directions at the same time; more and more particles farther and farther apart bound together in Einstein’s spooky embrace now known as “entanglement.” At the University of California, Santa Barbara, researchers are planning an experiment in which a small mirror will be in two places at once.

[div class=attrib]More from theSource here.[end-div]